The present invention relates to lifeline systems and, particularly, to self-retracting lifeline systems and braking systems therefore.
The following information is provided to assist the reader to understand the invention disclosed below and the environment in which it will typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the present invention or the background of the present invention. The disclosures of all references cited herein are incorporated by reference.
Many devices have been developed in an attempt to prevent or minimize injury to a worker falling from a substantial height. For example, a number of devices (known alternatively as self-retracting lifelines, self-retracting lanyards, fall arrest blocks, etc.) have been developed that limit a worker's free fall distance to a specified distance and limit fall arresting forces to a specified value.
In general, most currently available self retracting lifeline safety devices or systems include a number of common components. Typically, a housing or cover provides enclosure/protection for the internally housed components. The housing includes attached thereto a connector for anchoring the self-retracting lifeline to either the user or to a fixed anchor point. The connector must be capable of withstanding forces required to stop a falling body of a given mass in a given distance.
A drum or spool around which a lifeline is coiled or spooled rotates within the housing. The drum is typically under adequate rotational tension to reel up excess extended lifeline without hindering the mobility of the user. Like the anchor connector and the other operative components of the retractable lifeline safety device, the drum is formed to withstand forces necessary to stop a falling body of a given mass in a given distance. The lanyard or lifeline is attached at one end thereof to the drum to allow the drum to reel in excess lifeline. The lifeline is attached at the other end thereof to either the user or to an anchorage point, whichever is not already attached to the housing.
Self-retracting lifeline systems also include a braking mechanism which locks (that is, prevents rotation of) the drum assembly of the self-retracting lifeline upon indication that a fall is occurring. For example, when the safety line (for example, rope, cable or web) being pulled from the self-retracting lifeline system causes the drum assembly to rotate above a certain angular velocity, a brake mechanism can cause the drum assembly to suddenly lock.
Many currently available braking systems for self-retracing lanyard systems actuate upon the drum assembly reaching a predetermined angular velocity. The rotational velocity of the drum assembly is proportional to the linear velocity of the safety line. In the case of a self-retracting lanyard braking system which actuates at a predetermined or threshold angular velocity (such as that disclosed in U.S. Pat. No. 5,771,993), a pawl is typically attached to the drum assembly at a pawl pivot that is spaced from the center of gravity of pawl. The pawl can pivot relative to the drum assembly about the pawl pivot. A pawl spring applies a force tending to keep the pawl retracted against a pawl stop on the drum assembly. When the pawl is retracted, it cannot strike an abutment as the drum assembly rotates. As the drum assembly rotates, the center of mass of the pawl tends to follow a straight path tangent to the drum assembly, but the pawl is prevented from pivoting outward by the force of the pawl spring. If, however, the drum rotates at a sufficient velocity, the centripetal force required to keep the pawl against the pawl stop will exceed the force supplied by the pawl spring. At that point, the pawl rotates about the pawl pivot to a radially outwardly extended position wherein the pawl abuts an abutment (for example, on the housing) and brings the drum assembly (and the safety line) to a halt.
In designing a velocity actuated brake, the desired maximum or threshold safety line velocity (and a corresponding angular velocity of the drum assembly) must be defined. For example, the velocity or speed of a fast walk can be used. From the maximum safety line velocity, the maximum or threshold angular or rotational velocity of the drum assembly is determined. The centripetal force that must be supplied by the pawl spring is then determined from the mass of the pawl.
Braking systems based upon angular acceleration are, for example, commonly used in connection with automobile seatbelt restraints. Currently available acceleration braking systems typically include a system of low strength, complexly interacting parts and have not been widely accepted in the fall protection arts.
Although a number of braking mechanisms have been developed for use in connection with self-retracting lifeline and other systems, such mechanisms are often complex (for example, requiring a significant number of interconnected and often complexly operating components), relatively high in cost and insufficiently rugged.
It is thus desirable to develop systems, devices and methods that reduce or eliminate the above and other problems associated with currently available self-retracting lifeline systems.